There's an interesting, although dirty phenomena that occurs in the Caribbean each summer. It's called Sahara Dust.
Dust from the Sahara Desert is picked up by the wind currents moving off of Africa and deposited on anything it happens to pass as it moves westward. We started noticing it in May and if my memory of last season is correct it will continue for several months. We don't notice it on Merengue's deck until it rains. Then we find brown dirt marks where it collects under jerry cans. Where we notice it is on the leading edge of all the vertical surfaces like jib sheets, halyards and the roller furled genoa. Look how one side remains clean while the other is red-brown.